bugfix: add PCell to serving_cells in fallback path and refactor update logic

Fix assertion failure in rrc_gNB_send_NGAP_NAS_FIRST_REQ() when RRC Reestablishment falls back to RRC Setup. The fallback path was not adding the PCell to the UE's serving_cells array, causing rrc_get_pcell_for_ue() to return NULL.

Major changes:

• Add rrc_update_ue_pcell() helper function that handles PCell updates for both new UEs (fallback) and existing UEs (normal reestablishment)
• Refactor normal reestablishment path to use rrc_update_ue_pcell() helper instead of manual cleanup + add sequence
• Add PCell update to fallback_rrc_setup path using the same helper
• Move C-RNTI validation after current_cell fetch to ensure current_cell is initialized before fallback_rrc_setup label
• Change error handling: return instead of fallback when PCell add fails in normal reestablishment path

Fixes CI failure in !3908 (merged)